Fig. 2. Characterisation of X. laevis B-type cyclins. (A) Sequence alignment of cyclins B1, B2, B4 and B5; bars show the conservation, the destruction boxes are marked by a rectangle, and the nuclear export sequences indicated in red. (B) The dendrogram shows the family of mitotic cyclins in X. laevis and X. tropicalis. The scale bar represents 10 amino acid substitutions in the compared length of 180 residues. (C) Egg extracts were labelled with ³⁵S-labelled amino acids and immunoprecipitated with the indicated antisera. These immunoprecipitates were analysed by autoradiography and assayed for binding of Cdc2 and histone H1 kinase activity. (D) Immunoblot showing the destruction of each B-type cyclin after addition of CaCl₂ to an egg extract. (E) Cyclin levels during early embryogenesis determined by immunoblotting with the indicated antibodies. MBT occurred at about 10 hours in this experiment.